1. Field of the Invention (Technical Field)
The present invention relates to dampening of belt modes of stabilized mirrors.
2. Description of Related Art
Gimbal assemblies are commonly employed in optical systems, such as forward looking infrared (FLIR) and laser based targeting and imaging systems, to position and/or stabilize optics, including mirrors. Precise pointing and stabilization of the optical line of sight is important to ensure line of sight accuracy, modulation transfer function, and so forth. Known gimbal assemblies include mirrors and/or lens assemblies that are controlled by belt drive systems, wherein a belt couples a belt drive motor situated on a gyro platform with a rotatable mirror of the gimbal assembly.
Gimbal assemblies having belt drive systems are often used in applications where they are subjected to significant vibra-acoustic energy. Vibra-acoustic energy can disturb and/or degrade stabilization performance. In a gimbal assembly having a belt driven mirror, belt mode resonance causes amplification of mirror vibration that results in degradation of system performance. For example, mirror vibration can reduce the line of sight accuracy in a forward looking infrared radar/laser based optical system, further resulting in the degradation of targeting and/or imaging accuracy.
The vibra-acoustic energy causes the belt drive and mirror to vibrate at a frequency known as the belt modal frequency. For example, a belt modal frequency at around 400 hertz results in an approximate 52 decibel gain, which results in a substantial line of sight (LOS) disturbance of approximately 170 micro-radians root mean squared (RMS).
Most known systems use passive dampers to counter vibrations due to vibra-acoustic energy disturbance (e.g., vibra-acoustic disturbance torque). An exemplary passive damper can comprise a piece of tungsten steel mounted with rubber in shear, wherein the rubber has a high internal damping coefficient. Such passive dampers are also known in the art as inertial dampers or constrained layer dampers. However, passive dampers have several limitations. For example, passive dampers are limited in their ability to counter the effects of vibra-acoustic disturbance energy. Passive dampers typically provide only a 14 decibel improvement over undamped systems. Although passive dampers do provide some improvement, an undesirable amount of line of sight disturbance can still remain. In addition, passive dampers do not function efficiently, or at all, in cold environments. Notch filters may be employed to provide control loop stability but do nothing to attenuate LOS disturbance due to vibra-acoustics.
U.S. Pat. No. 6,137,254, entitled “Active Vibra-Acoustic Damper”, to Hughes, discloses one form of active damping, but lacks significant advantages of the present invention.